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Republican Scott P. Brown pulled off one of the biggest upsets in Massachusetts political history last night, defeating Democrat Martha Coakley to become the state’s next US senator and potentially derailing President Obama’s hopes for a health care overhaul.

The stunning, come-from-behind victory caps a dramatic surge in recent days as Brown, a state lawmaker from Wrentham once thought to have little chance of beating a popular attorney general, roared ahead of Coakley to become the first Republican senator elected from Massachusetts since 1972.

Coakley called Brown to concede a little more than an hour after polls closed, and the Brown campaign party erupted into jubilant cheers soon after.

“Scott Brown is the next United States senator!’’ Brown’s daughter Ayla shouted from the stage at the Park Plaza Hotel as the euphoric crowd began waving American flags.

In a race that became the focus of national attention, Brown’s win was widely seen as a vote against the president’s agenda from one of the most reliably Democratic states. In a particularly ironic twist, Brown may well be the 41st vote to prevent the Democratic-led health care plan from moving forward, though Edward M. Kennedy, who held the seat for 47 years, called health care “the cause of my life.’’

And while Brown paid homage to Kennedy, saying “his name would always command respect,’’ he spoke against the health care bill as the crowd chanted “Forty-one! Forty-one!’’

“I go to Washington as the representative of no faction or no special interest, answering only to my conscience and to you the people,’’ Brown said. “I know I have a lot to learn in the Senate, but I know who I am, and I know who I serve. I’m Scott Brown. I’m from Wrentham. And I drive a truck.’’

Brown’s strength came from the North and South shores, as well as the suburbs in the Interstate 495 belt, including Holliston, Ashland, and Marlborough, where successful Republicans have historically fared well. He ran up big margins in Plymouth and surrounding communities, as well as in Worcester County.

Coakley fared best in liberal suburbs immediately west of Boston, the Berkshires, and in cities such as Lawrence, Springfield, and New Bedford, though not by the large margins required to win statewide.

“I am heartbroken at the result,’’ Coakley told disappointed supporters at the Sheraton Boston. “But I know that we will get up together tomorrow and continue this fight, even with this result tonight.’’

Toward the end of her speech, her voice cracked as she quoted Kennedy, saying, “The work begins anew, the hope rises again, and the dream lives on.’’

President Obama called both candidates last night, congratulating Brown on a well-run campaign’’ and telling Coakley “we can’t win them all.’’

Coakley, after cruising to an easy victory in the primary, began the general election race with seemingly every edge, from name recognition and fund-raising ability to a lopsided advantage in voter registration and the backing of the state’s Democratic establishment. She had been plotting a race for US Senate for more than five years.

But Brown marched ahead in the two weeks after the holidays, channeling populist anger at Democratic policies in Washington and capitalizing on Coakley’s relatively low-key campaigning. He benefited from an influx of out-of-state activists and excitement among Massachusetts conservatives, who saw a rare chance to send a Republican to higher office.

Coakley’s loss is particularly dispiriting for the many women who were energized about the prospect of the state’s first woman senator.

Brown will seek to be sworn in as soon as possible, setting up an instant battle with Senate Democrats over when to seat him. Secretary of State William F. Galvin said he will send a letter with the unofficial results today to the secretary of the Senate, who has the authority to decide when to swear in the winner. (Galvin cannot officially certify the results for 10 days, until overseas absentee ballots are counted.)

“I’m ready to go to Washington without delay,’’ Brown said last night, with the crowd chanting “Seat him now! Seat him now!’’

National Republicans, who played a largely behind-the-scenes role, were giddy, while Democrats were morose.

“I have no interest in sugar-coating what happened in Massachusetts,’’ US Senator Robert Menendez of New Jersey, chairman of the Democratic Senatorial Campaign Committee, said in a statement.

Turnout overall was brisk, especially for a first-ever US Senate special election that followed a holiday weekend with snowy weather. More than 2.2 million people cast ballots, which matched the turnout in the 2006 governor’s race.

Brown’s election foiled efforts by Massachusetts Democrats to keep the US Senate seat in the party. The Legislature changed the law in 2004, fearing that Governor Mitt Romney would appoint a Republican if Senator John F. Kerry won the presidency. They changed the law again several months ago, allowing Governor Deval Patrick to temporarily appoint Paul G. Kirk to the seat. Kirk, a Democrat, will remain in place until Brown is sworn in.

Over the past two weeks, the contest between Coakley and Brown took on national implications, drawing outside groups who deluged voters with a flurry of television ads, automated phone calls, mailers, and e-mails, many of them negative.

Brown’s campaign courted voters with folksy ads from his kitchen and his GMC pickup. On the campaign trail, he frequently wore a barn jacket over his coat and tie. He often seemed taken aback by his newfound popularity after weeks of stumping at sparsely attended events in bars, diners, and train stations.

In a matter of weeks, that helped transform Brown from an obscure state senator to a giant-killer in the Republican Party. Even as he eschewed the Republican label at times, he reinvigorated the GOP base and has provided national Republicans with a template for the 2010 midterm elections.

Brown tapped into voter anger over both state tax increases and corruption on Beacon Hill, casting Coakley as an insider beholden to the Democratic establishment. And while Brown was embraced by right-wing groups, including activists opposed to gay marriage and abortion, he sought to portray himself as a moderate voice.

Brown supporters said they were put off by Coakley’s style and dismayed by the health care debate on Capitol Hill.

“I was disgusted by Coakley’s campaign,’’ said Tim Macinta, a 34-year-old software developer from Somerville. “She basically took the election for granted, and her ads were misleading.’’

After Kennedy’s death in August, Coakley was the first to announce her candidacy and was thefront-runner from the start. Following her 19-point primary victory last month, she seemed to have found the formula for success that had eluded previous women, including gubernatorial bids by former lieutenant governor Kerry Healey and former treasurer Shannon O’Brien.

But as Coakley stayed off the airwaves and shied away from meet-and-greet politicking, Brown gained notice by running television ads comparing himself to President Kennedy and defining himself as the champion of independent-minded voters.

Brown also, perhaps, capitalized on a sense among some voters that Democrats felt entitled to the seat the Kennedy family had controlled since 1952. In one of his more memorable lines in the final debate, Brown said, “With all due respect, it’s not the Kennedy seat, and it’s not the Democrats’ seat. It’s the people’s seat.’’